Broadly speaking, aerosol spray devices comprise a container holding a liquid to be discharged together and an outlet nozzle associated with a valving arrangement which is selectively operable to allow discharge of the liquid as a spray from the nozzle by means of the propellant provided within the container.
Both “compressed gas propellant aerosols” and “liquefied gas propellant aerosols” are known. The former incorporate a propellant which is a gas at 25° C. and at a pressure of at least 50 bar (e.g. air, nitrogen or carbon dioxide). Such a gas does not liquefy in the aerosol spray device. On opening of the valving arrangement, the compressed gas “pushes” liquid in the spray device through the aforementioned nozzle that provides for atomisation. There are, in fact, two types of “compressed gas propellant aerosols”. In one type, only liquid from the container (“pushed-out” by the compressed gas) is supplied to the outlet nozzle. In the other principal type, a portion of the propellant gas from the container is bled into the liquid being supplied to the nozzle which atomises the resulting two-phase, bubble-laden (“bubbly”) flow to produce the spray. This latter format can produce finer sprays than the former.
In contrast, “liquefied gas propellant aerosols” use a propellant which is present (in the aerosol spray device) both in the gaseous and liquid phases and is miscible with the latter. The propellant may, for example, be butane, propane or a mixture thereof. On discharge, the gas phase propellant “propels” the liquid in container (including dissolved, liquid phase propellant through the nozzle).
It is well known that “liquefied gas propellant aerosols” are capable of producing finer sprays than “compressed gas propellant aerosols”. This is due to the fact that, in the former, a large proportion of the liquefied gas “flash vaporises” during discharge of liquid from the aerosol spray device and this rapid expansion gives rise to a fine spray. Such fine sprays cannot generally be achieved with “compressed gas propellant aerosols”, in either of the two principal formats described above.
Attempts have been made to improve the “fineness” of sprays generated by “compressed gas propellant aerosols”. Prior art proposals have included the possibility of “bleeding off” some of the compressed gas (e.g. nitrogen) that is present in the container and mixing this with the liquid product to achieve “two fluid atomisation” which is a technique known to provide fine sprays for other areas of spray technology, e.g. liquid fuel combustion. However it has been found extremely difficult to produce fine sprays using two fluid atomisation with aerosol spray devices, and the nearest approach has been to use the equivalent of a vapour phase tap (VPTs are used in “liquefied gas propellant aerosols”) to bleed some gas into the valve. However results for improving spray fineness have not been significantly beneficial.
WO 90/05580 (Weston et al) discloses a discharge valve for regulating the flow of a liquid product from an aerosol canister pressurised by a permanent gas propellant such as nitrogen. The discharge valve incorporates a valve stem moveable from a first limit position to a second limit position to effect spray discharge from the device via a spray outlet region thereof. The valve stem is formed with a flow conduit (designated as a “mixing chamber”) formed with at least one upstream liquid orifice and at least one downstream gas orifice. The discharge valve incorporates a valving arrangement such that movement of the valve stem from its first to second limit position opens the liquid and gas orifices to cause a liquid/gas mixture to be produced in the mixing chamber. Table 1 of WO 90/05580 gives exemplary dimensions for the cross-sections of the mixing chamber, liquid and gas orifices but without detailed consideration as to the relative sizes thereof.
Downstream of the mixing chamber of the device of WO 90/05580 is at least one restrictor through which the mixture is forced to pass to produce a choked or sonic flow, resulting in the mixture expanding to form a “foamy mixture” which passes to an exit orifice at the spray outlet region for discharge from the spray device. The restrictors are employed in the aerosol spray device of WO 90/05580 to ensure that the spray from the exit orifice is essentially constant throughout the discharge of the liquid content of the device. This is achieved by ensuring that the residual pressure which remains across each restrictor when the liquid content of the device is about to become exhausted is still sufficiently high to produce at least substantially choked flow through the or each restrictor and thereby produce a shockwave after each restrictor. More specifically, the flow becomes supersonic downstream of the restrictor and shockwaves are produced as the flow subsequently goes from supersonic to subsonic resulting in vigorous break-up of gas particles to produce a uniform foam. However the need to provide the restrictors results in a relatively complicated construction and a discharge assembly which cannot readily be produced by mass-production techniques such as injection moulding.
Copending U.S. Patent Application No. 61/261,906 discloses an aerosol spray device for producing fine sprays in the case of “compressed gas propellant aerosols” although there is some applicability to “liquefied gas propellant aerosols”. Embodiments disclosed in the prior US application incorporate a spray discharge assembly incorporating a flow conduit for supplying fluid from a container to a spray outlet region of the device. The flow conduit has at least one first inlet for liquid from the container and at least one second inlet for propellant gas from a head space of the container. The spray discharge assembly further incorporates a valving arrangement such that movement of a valve stem from a first to second limit position opens the first and second inlets to cause a bubble laden flow to be generated in the flow conduit for supply to the spray outlet region. In accordance with the teaching of the prior US application, there is provided, downstream of the flow conduit, an approach chamber having at least one inlet and an outlet communicating with a discharge orifice. The outlet of the approach chamber (which is effectively an inlet to the discharge orifice) is surrounded by a sharp edge. Located between the flow conduit and the approach channel is at least one jetting orifice through which the bubble laden flow from the flow conduit passes and issues as a jet into the approach channel, the jetting orifice being configured for directing the jets against the sharp edge. The invention of the prior US application was found to produce fine sprays as a result of a separation and reattachment phenomenon of the bubble laden flow in the discharge orifice.